Telecommunications networks traditionally have used rings of optical fibers, where each optical fiber is able to carry a number of digital signals at different optical wavelengths. These fibers are connected to multiplexing and demultiplexing equipment that combine and separate the optical wavelengths. Although in the past these networks have been arranged in rings, higher equipment efficiency and lower equipment cost can be realized using networks arranged in a highly interconnected mesh. Typical mesh nodes might have four to six input and output fibers, each propagating approximately forty wavelengths. Optical switch sizes on the order of 256 input and output ports may be needed to interconnect wavelengths between these nodes, including providing capacity for adding and dropping dozens of signals at each node.
A method for optical wavelength multiplexing and demultiplexing to provide free-space collimated optical beams at separate wavelengths that interface directly with a free-space optical switch is discussed in Patel and Silverberg, Liquid Crystal and Grating-Based Multiple-Wavelength Cross-Connect Switch, IEEE Photonics Technology Letters, Vol. 7, pp. 514-516, 1995 (hereinafter “Patel”), using wavelength-dispersive media such as gratings to separate the optical beams from two input and two output fibers. The number of optical input and outputs ports can be increased over the wavelength dispersive switch method disclosed by Patel using an optical switch composed of a two-dimensional array of micromirrors between two gratings, as disclosed in U.S. Pat. No. 6,097,859 by Solgaard et al. However, short optical path lengths may be needed for robust packaging. Furthermore, wavelength-dispersive media such as gratings may require long optical path lengths to provide sufficient optical beam separation for the closely spaced wavelengths used in conventional telecommunications systems.
It is also difficult to fabricate mirror arrays with perfect yield, leading to blocking network operation in conventional configurations due to defective mirrors in the mirror array. Wavelength independent input and output ports of a wavelength selective switch may be needed in order to provide the ability to add and drop arbitrary wavelengths, as required in mesh telecommunications networks.